Previously, light boxes have been used for the purpose of interpreting X-ray films. Typically, such boxes have a white pane of glass or plastic which is illuminated from the rear and which transmits light in a diffuse manner. The X-ray film is placed upon the front of the pane for interpretation. In many cases, however, interpretation is difficult with such light boxes, particularly when the X-ray images suffer from poor contrast or are very bright, as often arises when using the minimum possible level of radiation to protect the patient. It is not a simple matter for a medical practitioner to explain a finding or diagnosis to a patient by means of X-ray images because such pictures do not provide a three-dimensional perspective for viewing. The typical patient has difficulty visualizing body structures from two-dimensional pictures. Three-dimensional or stereoscopic viewing provides a means for showing actual, more understandable spatial relationships and is thus the preferred method for explaining medical findings to a patient.
Improved interpretation of X-rays has been made possible by several types of light box accessories. Some accessories, in the form of magnifying lenses, enlarge a section of the X-ray so that the patient may more clearly see the portion of the X-ray of most interest. Contrast filters have also been used for darkening bright X-ray films so that two-dimensional viewing is improved. Magnifying lenses, however, inevitably produce distortion of the X-ray image and detrimentally affect the contrast thereof, while so-called contrast filters, such as tinted glass panes, which are placed over the X-ray film, only result in general darkening of the picture without actually improving the contrast between bright and dark portions of the X-ray image. Thus, magnifying lenses and contrast filters have not been an adequate solution to the general problem of the difficulty that doctors and patients have in visualizing and understanding X-ray images.
Stereoscopy was first introduced to radiology near the turn of the century and many radiographs were taken stereoscopically. One early stereoscopic technique comprised the steps of placing a photographic plate behind the patient, exposing the plate with the X-ray apparatus, replacing the plate, shifting the X-ray apparatus laterally, and taking a second exposure. The films developed from the two exposures were then viewed in a stereoscopic viewer apparatus. The additional expense and exposure to X-rays, with the discovery that overexposure to X-rays is harmful, led to a rapid decline in this type of stereoscopic X-ray technique.
Certain improvements in stereoscopic techniques were later made to reduce patient exposure or overexposure to X-rays. One such improvement was to use intensifying and fluoroscopic screens which emit light when the phosphors thereof are excited by X-rays. The light emitted from these screens exposes an emulsion on a photographic plate. In fluoroscopy, the fluorescent screen is viewed directly. Patient exposure is reduced by fluoroscopy, however, other problems surfaced. For example, fluorescent screens were too dim for daylight (photopic) vision which is sharper than night (scotopic) vision. X-ray image intensifiers were developed which absorbed X-ray photons and converted their energy into light photons. The light photons struck a photocathode causing it to emit photoelectrons which were drawn away by the high potential of an accelerating anode. An electrostatic lens focused the electrons onto an output fluorescent screen which emitted light viewed by an observer. Image intensifiers were also often coupled with motion picture, television, or spot film cameras for recording output fluorescent images. Interpretation of X-rays was still difficult using these methods. One reason for this difficulty was that the three-dimensional images were static, i.e., not moving and thus did not provide as much information as a moving image.
Stereoscopic imaging has been an area of both great interest and frustration. Although many techniques have been used to achieve the stereo effect, all share the common principle that an image recorded from the perspective of the right eye must be presented to the right eye and an image recorded from the perspective of the left eye must be presented to the left eye.
The simplest way to accomplish this is to provide distinct and separate optical paths to each eye from each recorded image. This is the principle behind the common hand-held three-dimensional stereoscopic viewer in which right and left eye image pairs are recorded as transparencies. The transparencies are mounted so that when they are inserted in a viewer, the images comprising a pair are presented to each eye separately through magnifying lenses. Recently a stereo camcorder (Toshiba) has been developed which records sequences of right and left eye image pairs which are played back through two small CRT displays which are mounted on a band which the viewer wears about the forehead. The screens are positioned to provide the appropriate image to each eye. This results in moving three-dimensional images.
Another system which uses the principle of distinct and separate optical paths is the mirror based viewer. In this system, right and left eye image pairs (either opaque prints or transparencies illuminated from below) are positioned under a viewer which, through two pairs of angled mirrors, directs each image of the image pair to the appropriate eye. This type of system has been used extensively to view stereo X-rays and was the method used in this current study.
Any system which uses distinct and separate optical paths to each eye has some disadvantages. Only one viewer can see the image at a time and the viewer must maintain his head in a relatively fixed and narrow position. To overcome these problems, left and right eye images have been combined for display and are then extracted and passed to the appropriate eyes just before the light passes to the viewer. Some systems using this approach are more suitable for projected images; others are more suitable for images presented on a cathode ray tube (CRT) monitor.
The earliest method of this type was to tint the right and left eye images with complimentary colors, typically green and red, and then to combine them during display. The viewers wore a green filter over one eye and a red filter over the other eye. The red filter blocked the green images and vice versa. Thus, each eye received only one image. The two images are combined in the brain and the result is an essentially monochroma stereoscopic three-dimensional view. This type of system can also work with some success to decode stereo images on a CRT monitor and has been the basis for some three-dimensional television broadcasts.
A similar approach uses polarizing filters. Right and left eye image pairs are polarized with linear (horizontal/vertical) or circular (clockwise/counterclockwise) polarizing filters. The images are combined for viewing and they are viewed through matching filters which pass the appropriate image to each eye. This method of stereo viewing permits color images to be projected in three-dimensions. However, it cannot be used for images on a monitor screen as CRT phosphors cannot reproduce polarized light.
There have been two recent developments which permit multiple viewers to see stereoscopic views on a CRT screen. The first is a system based on the use of lead lanthanum zirconate titanate (PLZT) electrooptic ceramics. This material can be used to produce a non-mechanical electrooptic shutter which can be opened or closed in response to an electrical signal. A pair of such shutters can be configured as glasses. These glasses can be controlled, in synchrony with the images passed to the monitor screen, so that the shutter to the left eye is opened (and the shutter to the right eye is closed) when the left eye image is on the screen and vice versa. This permits more than one viewer to observe the stereo images. Until recently, these systems required a wire between the glasses and the controller. More recent advances permit this communication to be accomplished with a wireless system.
A PLZT viewer based system is effective, but, even in its wireless embodiment, is an "active" system which requires a relatively complex and expensive controller for each pair of glasses. A more recent system (Tektronix Corporation) may solve this problem. A liquid crystal based polarizing filter is positioned over the CRT screen. The filter can be electronically controlled to alternate between the clockwise and counterclockwise polarizing states. The system can be timed so that the left and right eye images are alternately presented on the monitor; left eye images are subject to clockwise circular polarization and right eye images to counterclockwise circular polarization. The viewer wears glasses in which the left eye looks through a clockwise polarizing filter and the right eye looks through a counterclockwise polarizing filter. This type of system offers the distinct advantage that the glasses are a passive device which is inherently simple and lightweight.
By rotating an X-ray imaging device about a subject (or a subject under an X-ray imaging device) images acquired from differing perspectives suitable for use as binocular image pairs can be obtained. The common static stereoscopic X-ray viewer used mirrors to present the left and right X-ray images to the left and right eyes respectively. Inherent disadvantages of this technique included the requirement for two exposures (and therefore twice the X-ray exposure), the limitation to one viewer at a time, and the ability to view only static images. Static stereoscopic films are not commonly used in the current era.
It was also found that two fluoroscopic tubes could not be positioned close enough together so that images could be taken at angles less than 35 degrees between the tubes. Otherwise, the tubes would collide with one another. It has been determined, moreover, that the angular focal spot separation, which correlates with the effective viewing angle between two images, must be no greater than about 5 degrees in order to obtain accurate stereoscopic viewing. Accordingly, the problem encountered in the art for the production of stereoscopic images was that a small angle (less than 5 degrees) between the positions at which the X-rays are taken was required for proper stereoscopic viewing. Optimal focal spot separation is a function of many factors including interpupillary distance, magnification factor, viewing distance, and focal spot to imaging plane distance. As these are all variable, it is desirable to be able to vary the focal spot separation. Current stereofluoroscopes use a dual focal spot source housed in a single X-ray tube and the spacing of the focal spots is fixed.
Dual focal spot stereo fluoroscopy systems overcome the limitation that two separate X-ray tubes cannot come close enough together. These systems permit motion to be recorded by generating and recording sequences of image pairs with a dual X-ray source. Such systems have been used principally for neuroradiology procedures. Early systems displayed these image pairs on two monitors and combined the images with a mirror based stereo image combining device (with its inherent disadvantages, as noted above). Later devices have combined the images with a PLZT electrooptic shutter system, allowing multiple users to see the images without being confined to the eye ports of a viewing device.
This clearly represented an improvement. However, the following disadvantages remain:
(1) Dual focal spot fluoroscopes are special purpose instruments which are not commonly available.
(2) These devices have a fixed distance between the two radiographic sources. As pointed out by Takashi et al. optimal focal spot separation (S) is a function of interpupillary distance (S.sub.V), magnification factor (M), viewing distance (D.sub.V), focal spot to film plane distance (D), and object size. Assuming that the object size is small compared to D and D.sub.V, this relationship may be expressed as follows: ##EQU1##
Because these dimensions vary from study to study and from viewer to viewer, a compromise value for focal spot separation (on the order of 3 cm) has been used. Furthermore, depth perception is highly variable between individuals.
(3) A dual focal spot device has two X-ray sources and delivers double the radiation of a single source, i.e., a single focal spot X-ray device.